[WIP] Add support for unstable WebIDL (#1997)

* Re-enable WebGPU WebIDL as experimental

* Add `web_sys_unstable_apis` attribute

* Add test for unstable WebIDL

* Include unstable WebIDL in docs.rs builds

* Add docs and doc comment for unstable APIs

* Add unstable API checks to CI

crates/webidl-tests/main.rs

@@ -13,3 +13,4 @@ pub mod namespace;
 pub mod no_interface;
 pub mod simple;
 pub mod throws;
+pub mod unstable;